I think baseball has it the hardest if you factor in recruiting. You have to be able to sift through all the really good prospects to find the ones that won't get drafted high enough to go straight into pro ball.

Basketball has a similar problem in that it is hard to build a team if the best kids leave after a year.

At least in football you are guaranteed 3 years with most players. This allows the best teams to recruit well, and build around key players.